This invention relates to a novel device for the continuous measurement of the ammonia concentration (NH.sub.3) in gases, for example, flue gases, in the ppm range. A method for the measurement of ammonia concentration is also described, which method can be carried out with the device of this invention.
The quantitative determination of the ammonia concentration in a gaseous media has become important due to the introduction of secondary measures for the removal of nitrogen (i.e. denitrofication) from furnace systems. In most instances, the process of selective catalytic reduction (SCR) with ammonia as the reducing agent is used for lowering the nitrogen oxide content of the flue gases. The continuous measurement of ammonia content is desirable for controlling such systems and for monitoring the prescribed emission values of the gases.
Devices for measuring the concentration of ammonia are currently known which are based on various methods and principles and are suitable for various measurement ranges.
A distinction can be made between methods which detect the NH.sub.3 directly in the gaseous phase and those in which NH.sub.3 is chemically bound and then determined in solution. Methods in which the NH.sub.3 concentration can be determined directly in the gaseous phase are usually based on the photometry of the NH.sub.3 in the infrared (IR) range. In these methods, the intensity of light (preferably infrared light) absorbed in, transmitted by, or reflected from a gaseous sample may be quantitatively measured and compared against a reference light intensity, to thereby provide a quantitative measurement of ammonia concentration in the gaseous sample. In these methods, specimen gas is removed from a flue-gas line via a gas probe and conducted into a cuvette of a photometer. The lowest measuring range of most of the commercially available devices is in the range of about 0-300 ppm.
The range between 0 and 10 ppm ammonia, which is, for example, downstream of the catalyst and particularly interesting in the monitoring of denox systems (i.e. systems for freeing flue gases from NO.sub.x, according to Ernst), cannot be reliably determined by the devices and methods described above. However, laser photometers have become commercially available wherein NH.sub.3 concentrations around 1 ppm can be determined in situ. However, these laser devices are extremely expensive, require a long absorption stretch and exhibit high variations of errors.
Another method for determining the NH.sub.3 concentration in a gaseous phase is based on the combustion of the NH.sub.3 on a catalyst and then measuring the heat of the reaction. This method is reliable only at higher concentrations of NH.sub.3 and in the absence of further oxidizable substances, for example, carbon monixide and hydrocarbons.
Methods in which NH.sub.3 is chemically bound from the gaseous phase and the concentration of NH.sub.3 then determined in solution are used more frequently, since these methods are more sensitive. However, these methods have the disadvantage of being more time-consuming and are usually operated in a discontinuous manner.
In addition, calorimetric methods are often used in which a quantitative measurement of the light absorption of a dye material which arises in a color reaction with NH.sub.3 and other reagents is used to determine concentration of ammonia.
Other frequently used methods for determining ammonia concentration are ion-sensitive potentiometry and the conventional wet-chemical determination of NH.sub.3 per acid-base titration after a Kjeldahl distillation. However, these methods are also discontinuous and usually require a considerable amount of time until the complete result of the analysis is available. Other, less frequently used methods make use of the photoionization of NH.sub.3 and/or the use of electrochemical sensors. however, the desired measuring range between about 0 and 10 ppm ammonia cannot be achieved by these methods.